1. Field of the Invention
The present invention relates to an electric power steering apparatus.
2. Description of Related Art
An electric power steering apparatus has an electric motor for generating a steering assist force and a reduction gear mechanism for decelerating the rotation of an output shaft of the electric motor. The reduction gear mechanism has a worm serving as a driving gear connected to the output shaft of the electric motor and a worm wheel serving as a driven gear meshed with the worm. The worm wheel is fixed to a steering shaft. The steering shaft is rotatably supported by a bearing. The bearing is held in a bearing holding hole of a housing (see Japanese Unexamined Patent Publication No. 2005-280589).
In the electric power steering apparatus, a proper amount of backlash is usually provided between the worm and the worm wheel at the time of factory shipment. When the electric power steering apparatus is employed for a long time period, however, the inner diameter of the bearing holding hole is enlarged. As a result, a clearance may occur between the bearing and the bearing holding hole. When the clearance occurs, the amount of backlash between the worm and the worm wheel is increased. As a result, a tooth striking sound is increased, so that noises become a problem.
When the amount of thermal expansion of the worm wheel due to temperature rise is larger than the amount of thermal expansion of the housing, the amount of backlash is decreased as the temperature rises. Therefore, a rotational resistance between the worm and the worm wheel is increased, resulting in a degraded steering feeling.
An object of the present invention is to provide an electric power steering apparatus capable of maintaining low noises for a long time period and superior in a steering feeling irrespective of temperature changes.